Add support for DateTime.strptime

[thedarkone]

Full disclosure: basically, when it comes to Time/Date/DateTime, I don't really know what I'm doing.

For the implementation I copied Date's strptime code.

[tamird]

Why is this code necessary? doesn't this work anyway?

[thedarkone]

If I removed the DateTime's strptime patching, the test would fail (DateTime.strptime('04-14', '%m-%d') would use the current year instead of 1984). Am I missing something?

[travisjeffery]

@thedarkone i just tried your code and your test fails even with your change